This invention relates to the production of aluminum alloy products and, more specifically, to an economical, effective and high productivity process for making high strength aluminum foil.
Aluminum foil is produced from a number of conventional alloys. Table I below lists nominal compositions and typical properties for annealed foils produced from typical Aluminum Association (AA) alloys.
One method of producing the foil is first to cast an ingot by a process commonly referred to as direct chill or DC casting. Foil made of 8006 alloy is typically produced by the DC casting process. The DC cast ingot is preheated to a temperature around 500xc2x0 C. and then hot rolled to produce a sheet having a thickness of about 0.2 to 0.38 cm (0.08 to 0.15 inches). This sheet is then cold rolled to a final thickness of 0.00076 to 0.0025 cm (0.0003 to 0.001 inches) to produce a household foil. During the process of cold rolling, the sheet work-hardens, making it impossible to roll it down further once a gauge of 0.005 to 0.010 cm (0.002 to 0.004 inches) is reached. That is why, after a few cold rolling passes (generally at a thickness of 0.005 to 0.05 cm (0.002 to 0.02 inches)), the sheet is interannealed, typically at a temperature of about 275 to about 425xc2x0 C., to recrystallize and soften the material and ensure easy rollability to the desired final gauge. The thickness of the sheet is normally reduced by about 80 to 99% after the interanneal. Without this anneal, work-hardening will make rolling to the final gauge extremely difficult, if not impossible.
The final gauge may be about 0.0008 to 0.0025 cm (0.0003 to about 0.001 inches). A typical final gauge for household foil is 0.0015 cm (0.00061 inches). When cold rolling is finished, the foil is then given a final anneal, typically at about 325 to 450xc2x0 C., to produce a soft, xe2x80x9cdead foldxe2x80x9d foil with the desired formability, and wettability. (xe2x80x9cDead foldxe2x80x9d is an industry recognized term for foil that can be folded 180xc2x0 back upon itself with no spring back.) The final anneal serves the purpose of imparting the dead fold characteristics as well as ensuring adequate wettability by removing the rolling oils and other lubricants from the surface.
Foil is also produced with other alloys such as 1100, 1200, 8111 and 8015 that is first cast as a sheet on continuous casting machines such as belt casters, block casters and roll casters. Continuous casting is usually more productive than DC casting because it eliminates the separate hot rolling step as well as the soaking and preheating step and scalping of the ingot. Continuous casting machines such as belt casters are generally capable of casting a continuous sheet of aluminum alloy less than 5 cm (2 inches) thick and as wide as the design width of the caster (typically as much as 208 cm (82 inches)). The continuous cast alloy can be rolled to a thinner gauge immediately after casting in a continuous hot or warm rolling process.
Typically, as with DC cast material, continuously cast sheet receives one interanneal and one final anneal. For example, the alloy may be cast and hot or warm rolled to a thickness of about 0.127 to 0.254 cm (0.05 to 0.10 inches) on the continuous caster and then cold rolled to a thickness of about 0.005 to 0.05 cm (0.002 to 0.02 inches). At this stage, the sheet is interannealed to soften it and then it is cold rolled to the final gauge of 0.00076 to 0.00254 cm (0.0003 to 0.001 inches) and given a final anneal at a temperature of 325-450xc2x0 C.
As may be seen from Table I, foils having significantly higher strength than standard household foils (conventionally produced with alloys such as 1100, 1200 and 8111) can be produced from certain currently available alloys, such as DC cast alloy 8006 and continuously cast alloy 8015. Unfortunately, both of these materials create certain problems. As mentioned above, the DC casting process used with alloy 8006 is relatively expensive. However, continuously cast 8015 is very difficult to roll and cast. Recoveries are poor, both during casting and rolling, because of problems such as edge cracking. The excessive work hardening rate results in lower rolling productivity due to increased number of passes required thereby increasing cost. This eliminates most if not all of the cost advantages of continuous casting.
The high iron content in both 8006 (1.2-2.0% Fe) and 8015 (0.8-1.4% Fe) is another problem. Alloys with this level of iron cannot be recycled with valuable low iron alloysxe2x80x94the predominant example being beverage can sheetxe2x80x94without blending in primary low iron metal to reduce the overall iron level in the recycled metal. As a result, alloys such as 8006 and 8015 are sometimes unacceptable for recycling. If they are accepted at all, it may only be with a cost penalty. Additionally, high iron contents make these alloys difficult to cast and to roll into foil.
Japanese patent publication number 62149838 filed on Feb. 2, 1986 by Showa Aluminum Corporation of Japan discloses an aluminum alloy foil having good formability. The foil is produced by subjecting the alloy containing specific amounts of Fe and Mn to homogenizing treatment, hot rolling, and then to cold rollings with interposing process annealing between the cold rolling steps. The interannealing is carried out at 400xc2x0 C. for one hour.
According to one aspect of the present invention, there is provided a process of producing aluminum foil having dead fold foil characteristics with a yield strength of at least 89.6 MPa (13 ksi), and ultimate tensile strength or at least 103.4 Ma (15 ksi) and a Mullen rating of at least 89.6 kPa (13 psi) at a gauge of 0.0015 cm (0.0006 inch), wherein an aluminum alloy is cast to form an ingot or continuous sheet, the ingot or continuous sheet is cold rolled to produce a cold worked sheet, the cold worked sheet is interannealed, the interannealed sheet is cold rolled to a final gauge sheet of foil thickness, and the final gauge sheet is annealed. in the invention, the aluminum alloy is selected to contain an amount of manganese in the range of 0.05 to 0.15% by weight, and the cold worked sheet is interannealed at a temperature in the range of 200 to 260xc2x0 C.
This invention provides a process of producing a high strength aluminum foil with mechanical properties comparable to foils made of 8006 or 8015 alloys, without the difficulties and cost penalties associated with the production and rolling of 8006 and 8015 alloys. The process may be used with a number of alloys that are relatively easy to cast and roll with good recoveries (typically rolling recoveries are about 80%). The invention is most preferably carried out with alloys having low iron contents (i.e. less than about 0.8% by weight, and preferably 0.1 to 0.7% by weight) since higher iron contents make casting and rolling more difficult, and make the resulting scrap more expensive to recycle. Thus, foils made with this process can be produced relatively easily and recycled without cost penalty.
The invention requires that the manganese content of the alloy be between about 0.05 and about 0.5%, preferably about 0.1% to about 0.12%, by weight. We have found that foils with properties matching those of 8006 or 3015 foils can be produced, with superior recoveries and other operating advantages, by controlling the manganese level within these ranges and controlling the interanneal and optionally the final annealing temperatures.
As with previous processes for producing foil, sheet produced in the processes of this invention is interannealed, typically after one to three cold rolling passes. The process of the present invention differs from conventional techniques, however, by maintaining the annealing temperatures at relatively low levels that control the amount of manganese that precipitates from the alloy. We have found that manganese precipitation can be controlled by controlling the interanneal temperature. This controlled precipitation produces an interannealed sheet that can be rolled to final gauge with good recoveries, and produces a finished foil with superior mechanical properties.
The interannealing temperature is maintained at a level that will cause substantially complete recrystallization of the cold worked sheet without causing unacceptable precipitation of manganese. The interannealing temperature in the process of the present invention is preferably about 200 to 260xc2x0 C., and more preferably between about 230 and about 250xc2x0 C. The annealed sheet will contain at least about 0.05%, preferably at least 0.08%, and even more preferably about 0.09% to about 0.12% manganese in solid solution, where it can have the greatest impact on the mechanical properties of the finished foil.
Final annealing temperatures are also preferably controlled, and are matched to the interannealing temperatures and manganese content of the alloy to achieve the best balance of mechanical properties and processing characteristics. As with the interannealing temperatures, the final annealing temperatures are significantly below the annealing temperatures utilized in conventional foil production processes. In the processes of the present invention, the final annealing temperature is preferably about 250xc2x0 C. to about 325xc2x0 C., and more preferably between about 260xc2x0 C. and about 290xc2x0 C. With the levels of manganese that remain in solid solution following interannealing, the final gauge sheet can be finally annealed at these temperatures to produce a soft, formable foil, wish the dead fold characteristic that is very much desired in an aluminum foil, while still retaining strength and other mechanical properties equivalent to 8015 foil.